Water and ice polymorphs

Water and ice polymorphs. Water is a very interesting compound as the oxygen quadrupole coupling tensor of the free molecule has been accurately measured by beam-maser spectroscopy (27) and various ice polymorphs have been studied by NMR or NQR (28). This gives us an opportunity to show that the PCM formalism is able to switch easily between isolated molecules and fully reticulated networks. Table II gives our results obtained using the following parametrization for... 

N. Kumagai, K. Kawamura, andT. Yokokawa, Mol. Simul., 12,177 (1994). An Interatomic Potential Model for H2O Applications to Water and Ice Polymorphs. 

It is not appropriate in this chapter to give a detailed review of the solid-state behaviour of water in its various crystalline modifications. However, there are some general structures which are relevant and worth highlighting. Firstly, water molecules in these various solids have dimensions and bond angles which do not differ much from those of an isolated water molecule. Secondly, the number of nearest neighbours to which each individual molecule is hydrogen-bonded remains four, regardless of the ice polymorph. 

The effects of pressure on the phase transition of liquid water to ice (and within the ice phase itself) are complicated by the formation of several pressure-dependent ice polymorphs (Chaplin, 2004 Franks, 1984, 2000 Kalichevsky et al., 1995 Ludwig, 2001). Thirteen crystalline forms of ice have been reported to date Ih (hexagonal or normal or regular ice), Ic (cubic... 

The static permittivity, e, of ice at 273 K is higher than that of water (Whalley et al., 1966 Wilson et al., 1965). However, e -values for two polymorphs, ice-II and ice-VIII, stand out as being very small and in the range expected for non-polar systems. These polymorphs... 

Kamb B (1968) Ice polymorphism and the structure of water. In Rich A, Davidson N (eds) Structural chemistry and molecular biology. Freeman, San Francisco, pp 507-544... 

Some indirect experimental evidence exists for the liquid-liquid critical point hypothesis from the changing slope of the melting curves, which was observed for different ice polymorphs (30, 31). A more direct route to the deeply supercooled region, by confining water in nanopores to avoid crystallization, has been used more recently by experimentalists. These researchers applied neutron-scattering, dielectric, and NMR-relaxation measurements (32-35). These studies focus on the dynamic properties and will be discussed later. They indicate a continuous transition from the high to the low-density liquid at ambient pressure. The absence of a discontinuity in this case could be explained by a shift of the second critical point to positive pressures in the confinement. This finding correlated with simulations, which yield such a shift when water is confined in a hydrophilic nanopore (36). 

At high pressure, there are many kinds of ice polymorphs and the phase diagram of water is complicated. In ice VlII and XI, protons are ordered while most of ice phases have proton disordered forms. In ambient condition, satisfied is the ice rule water exists as an H2O molecule and a proton sits between two adjacent oxygens. In ice Ih, the number of configurations arising from the proton-disordering is approximately (3/2) " for Nw molecule system[16. This is also true for ice Ic and some other ices except for proton-ordered forms. 

We have seen that the structures of the ice polymorphs and of the ice-like hydrates indicate that the H2O molecule behaves as if there is a tetrahedral distribution of two positive and two negative regions of charge. The arrangement of nearest neighbours of water molecules in many crystalline hydrates is consistent with this tetrahedral character of the water molecule. In hydrated oxy-salts we commonly find a water molecule attached on the one side to two 0 atoms of oxy-ions and on the other to two ions or to one ion thus ... 

Two water molecules connected by hydrogen bond in hexagonal ice can have four possible configurations. These configurations are distinguished by the relative orientations of the two molecules and termed for obvious reasons as c-cis, h-cis, c-trans, and h-trans. (See Figure 2) Occurrence of symmetry permitted dimer orientations is a characteristic feature of each ice polymorph. In the proton-ordered structures the occurrence of orientations is... 

L. Mercury, P. Vieillard Y. Tardy (2001). Appl. Geochem., 16, 161-181. Thermodynamics of ice polymorphs and ice-like water in hydrates and hydroxides. 

Otero et al. (1998) demonstrated that high-pressure frozen eggplant samples had higher firmness and lower rupture strain and drip loss compared to those of still-air-frozen samples. The improved quality was attributed to the formation of heavy ice polymorphs due to freezing of water under high pressure (100-700 MPa), leading to volume reduction. 

Apart from the ice polymorphs, other types of solid but non-crystalline water exist, and they apparently form the most abundant state of H2O in the universe, where they are found adsorbed onto interstellar dust particles that eventually form comet tails. Amorphous, glassy water is mentioned here only for the sake of completeness, because its properties... 

Kamb, B. (1968). Ice polymorphism and the structure of liquid water. In Structural Chemistry and Molecular Biology, ed. A. Rich and... 

Much work has been done to characterize the structure and properties of all the ice polymorphs. For the purpose of understanding liquid water and its role in biological systems, one does not need to know the details of the high pressure forms of ice. There is one property worth noting, however, which is relevant to the study of liquid water. This has to do with the densities and lattice energies of the high pressure ices. 

The phases of a one component system are usually presented on a / , li diagram. Figure 1-15 shows ap, t diagram for water. The diagram is divided into three areas, the solid phase (ice), the liquid phase (water), and the vapor phase (steam). At a higher pressure, due to the existence of additional solid phases, further regions are added (polymorphic). Inside each area the system is divariant pressure and temperature may be varied independently. 

Figure 5.7 High-pressure phase diagram of water showing various polymorphs of ice. Additional polymorphs of ice (Ic, fV, Vll-Xin) exist at higher pressures (or lower temperatures than shown here). Note Only the hqnid boundary with ice-Ih has negative slope. The liquid boundaries with ice-IH, ice-V, ice-VI, and ice-Vn aU have positive slope.

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